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HDC3020: Design help to measure temp at another location

SndrS
Prodigy 75 points
Part Number: HDC3020

Tool/software:

Dear all,

We are in de midst of (re)designing a solution that includes the HDC3020. However we need to measure the temp/humidity at a different (physical enclosed) location of the device. At another location than the PCB with the sensor in, max 30mm away. 

In the old design there is a separate, second PCB that contains this sensor.

We are thinking to add a 'steel rod' (or pipe) to guide/conduct/transport the temperature from the other 'room' and bring it close to the HDC3020 to the top or to the pad in the middle. 

Any suggestions? Or can you recommend another approach (or reference article) that helps to design this?

Thanks in advance.

  • 0
    TI__Intellectual 1845 points

    Hello,

    Can you tell me a little more about your application and what exactly you are trying to measure? Do you want to measure the ambient conditions, or a board/component temperature? Do you expect a large temperature gradient within the system? 

    To measure a component temperature, generally we would recommend to place the temperature sensor as close as possible to the heat source that you want to measure, and ensure there is good thermal conduction between the heat source and the temp sensor. This may be a little difficult considering the layout is already set in your application, but maybe the steel rod/pipe might help with this.

    I would be concerned about the achievable humidity accuracy with this setup though. Since relative humidity will change with temperature, if there are temperature gradients/fluctuations within the system, this can introduce RH measurement error. To measure ambient air RH and temperature, we recommend to design for a low thermal conductivity between the PCB and the sensor, and ensure the sensor is exposed to the true ambient conditions and not affected by any heat sources. For example, when ambient conditions are 50%RH and 25C, if the sensor is heated up by even +1C then this will cause a -3%RH measurement error. 

    Best regards,

    Sakeenah 

  • 0 in reply to Sakeenah Khan
    Prodigy 75 points

    Hi Sakeenah,

    Thanks for your reply. Yes, I understand it's a bit hard to understand from the little information provided. Our application is a device that measures certain samples in a different (enclosed) compartment. Outside that compartment lays the main board. The expected temperature range will be +/- 10 degrees of room temp. (~15-35C). Temperature is a very critical factor for that chamber. Measuring mainboard temperature happens already.

    Yes, you are correct. The humidity is the main concern with this steel-rod/pipe approach. Thinking of this: there seems no other approach than placing the sensor as close as possible to the sample inside that other compartment. Unless you are having some great design suggestions/alternatives?

    btw: Are you aware of any (enclosed) wired versions of this sensor (alone)?

    Thanks.

  • +1 in reply to SndrS
    TI__Intellectual 1845 points

    Hello,

    Unfortunately I agree that humidity accuracy is a big concern. I'm still not sure which humidity you are trying to measure:

    • If you need to measure the humidity inside the different (enclosed) compartment, you need to ensure airflow and temperature uniformity between the compartment and the humidity sensor.
    • If you need to measure the outside (ambient) humidity, but also want to measure the inside compartment temperature (by creating a thermal path to the compartment), then that temp delta is going to mess up the ambient RH measurement. It is better to have a separate temp sensor dedicated to measuring the compartment temp. 
    • Either way, with the current setup and 10C delta, there will be significant measurement errors introduced. You can try doing some measurement compensation (which I'll describe below) however I fear it will not fully compensate due to the large temp delta.

    How to compensate RH measurements when there is heating or temp gradients: 

    • Relative humidity (RH) is dependent on temperature, pressure, and absolute humidity. Heating causes an "artificial" negative RH error and negative gain, because the HDC sensor no longer experiences the ambient temp/humidity due to heating.
    • You can estimate the amount of expected RH shift caused by temp deltas by calculating the dew point.
    • As an example, if you know the heated HDC sensor's temperature and humidity reading, and you know the ambient temperature (from a reference or another temp sensor), 
      • Step 1: use the HDC temp and HDC humidity to calculate the dew point.
      • Step 2: use the dew point and the known ambient temperature to calculate the ambient RH.
      • Equations are available in the 3rd content recommendation, and you can also find some calculators online pretty easily to calculate the dew point or RH. 
      • Please note this may not eliminate all test error especially when there are large temp deltas/gradients/fluctuations in your system.

    Recommended content:

    "Are you aware of any (enclosed) wired versions of this sensor (alone)?" -> I'm not sure I understand, do you mean like a wireless version that could exist inside of the enclosed compartment but still communicate with the main board? All the HDC302x sensors come in the same package for now, and all require connections for VDD/GND/SCL/SDA. I think you could potentially use some kind of bluetooth module for wireless communication, however of course the sensor still needs power.

    Best regards,

    Sakeenah 

  • 0 in reply to Sakeenah Khan
    Prodigy 75 points

    Hi Sakeenah,

    Thanks for your message and recommended content links. 

    I feel with your answer has enough information to continue our design part. Thanks. 

    Best, Sander

  • 0 in reply to SndrS
    TI__Intellectual 1845 points

    Hi Sander,

    Happy to support! Please feel free to open a new ticket if you have any additional questions/issues. 

    Best,

    Sakeenah